1. Technical Field
The present invention relates to a semiconductor light source lighting circuit for turning on a semiconductor light source such as an LED (light-emitting diode).
2. Related Art
In recent years, LEDs which have longer life and lower power consumption than conventional halogen lamps which use filaments have come to be used in vehicular lamps such as headlights in place of halogen lamps. The degree of light emission, that is, the brightness, of the LED strongly depends on the current flowing through it. Therefore, to use LEDs as a light source, a lighting circuit for adjusting the current flowing through the LEDs is necessary. Usually, such a lighting circuit has an error amplifier and performs a feedback control so as to keep the current flowing through the LEDs constant.
For example, in the case of headlights, to realize both of a high-beam mode and a low-beam mode properly and to satisfy a standard more easily, it is desirable that the brightness of LEDs be adjustable. Two methods for changing the brightness of LEDs are known which are a method of changing the current value continuously and a PWM (pulse width modulation) dimming method of changing the on/off duty ratio of a current. The former method has a color shift problem that the hue or the color temperature may vary depending on the current value. Therefore, in many cases, LED lighting circuits for vehicular lamps employ the latter, PWM dimming method.
The present applicant proposed a lighting control device which employs PWM dimming (see e.g., JP-A-2010-170704).
In the lighting control device disclosed in JP-A-2010-170704, the value of an LED current that was detected during a drive period of a switching regulator is held in an analog manner using a capacitor in a suspension period that follows the drive period. However, in general, the capacitor has a loss and hence the voltage held by the capacitor varies gradually. To restore an LED current value before a suspension period when a transition is made from the suspension period to a drive period, it is necessary to return a voltage that has varied in the suspension period as mentioned above to an original value. However, in general, the voltage of the capacitor varies more slowly than the LED current rises. Therefore, the LED current may overshoot, that is, it may reach a targeted value before the voltage returns to the original value and exceed the targeted value.
Similar phenomena occur in cases other than the PWM dimming. When the input voltage of a lighting control circuit or the number of LEDs to be driven is changed suddenly, the error amount in a current feedback loop may not be able to respond to such a sudden change properly, possibly resulting in an overshoot or undershoot of the LED current.